(1) Field of the Invention
The invention relates to a magnetostrictive position sensor and to a method for its operation.
(2) Description of the Related Art
In a magnetostrictive position sensor a magnetostrictive and electrically conductive wave conductor extends along the measuring range relative to which the position of the position indicator is to be determined. The position indicator is particularly a permanent magnet.
As it is well known, the position is determined through an electrical impulse being imparted into the wave conductor from an input position, which generates a mechanical-elastic wave at the position of the magnetic position indicator through its magnetic field, which propagates from the position of the position indicator along the wave conductor, also in the direction of a measurement location on the wave conductor, typically one end of the wave conductor.
Through the measurement of the time interval between imparting the electrical impulse at the input position—which is typically identical with the measurement location—and the arrival of the mechanical-elastic wave at the measurement location, which is converted into an electrical impulse at this location, from the runtime—thus the sum of the run time of the electrical impulse to the position indicator and the reverse runtime of the mechanical-elastic wave to the measurement location—in consideration of the known propagation speed of the electrical impulse, as well as of the mechanical-elastic wave along the wave conductor, the distance of the position indicator from the measurement location, and thus the position of the position indicator relative to the wave conductor, and thus also relative to the entire positions sensor can be determined, and thus with an accuracy in the single digit the μ-meter range.
It is already known from the basic patent with respect to the magnetostrictive position determination (U.S. Pat. No. 3,898,555A by Tellerman) to determine the time interval through a clock, timed in the MHz range, for example a quartz clock, by counting the number of cycle periods digitally. The more precise the position determination has to be performed, the higher the resolution of the timing of the clock has to be, which is used for time measurement.
The faster the timing of such time measurement unit, the higher is also the energy consumption and the generation of heat, especially when the fast timed time measurement unit is to run for the entire duration to be measured, thus from the start impulse to the stop impulse. Since energy consumption as well as the generation of heat generated thereby in the electronic components is disadvantageous for the measurement result, it is attempted to avoid this effect.
For this purpose, it is suggested in the EP 0885373B by the MTS company to use the so-called SARA-assembly to measure the time interval to be measured through a coarsely timed coarse clock, operating in the MHz range, and to measure the fine time difference resulting from the integer periods of the coarse clock and the time interval to be actually measured, with a fine timing device, timed faster and operating in the GHz range, and to compute the total time from the coarse time value and the fine time value.
Supposedly, this has the advantage that the fine timing device has to run only during the substantially smaller residual time interval and not during the entire measuring time interval, and thus its resultant energy consumption and the generation of heat in the processing electronics is minimized.
In this publication, the coarse counter is only started at the beginning of the time interval to be measured, which is a disadvantage, since the transient response of the coarse counter at start-up induces imprecision's into the measurement result.
From the DE 19703633C patent, it is known to provide a system timing for the purpose of coarse counting, which is already active before the beginning of the time interval to be measured, and which runs in particular for the whole operating duration of a respective position measuring device.
This, however, is causing the disadvantage that at the beginning and also at the end of the entire measuring time interval residual times with a duration below one period of the system timing of the coarse time remain, which need to be determined, which, in this state of the art document, is done through charging capacitors, whose charging state reached during the residual times is determined analogue and converted into a fine time value.
Thereby, a fine time clock, which is timed faster than the relatively coarse system timing, can be completely dispensed with. However, the complexity to determine the total time is relatively high with respect to the computation effort and also with respect to the respective device.